The technology of using gratings to analyze the wavelength content of incident light dates back to about 1786, when the American astronomer David Rittenhouse made a grating by arranging fifty or sixty hairs in the threads of a pair of miniscule brass screws at a pitch of 106 to the inch. Rittenhouse noted his surprise that red ray paths were bent more than blue ray paths and that a single slit source generated three parallel lines of light.
About a century later, Lord Rayleigh recorded the first holographic diffraction grating and noted its properties. However, classical ruled gratings remained the standard until the 1960's when Flamand applied a heuristic approach to solve the problem of the aberrations in reflective diffraction gratings. This made possible the now extensive use of aberration corrected holographic gratings that we have today.
More recently, the availability of computers of previously almost unimaginable power, has resulted in the development of numerous low aberration grating designs. These are made by using the computer to simulate the recording of the grating at various recording points. By trial and error, exploring variations of recording parameters about known or intuitively favorable starting points, one simulates the manufacture and evaluation of extremely large numbers of gratings. This information determines a high quality design for a particular purpose.
As noted above, the concave holographic aberration corrected grating is extensively used today. Its advantages lie in the simplicity of the mounting system and the relatively low cost of the commonly used grating replicas, which are formed by a molding process from a photographically recorded grating original. Considering the mounting, there have been numerous mountings proposed over the years, each of which has its particular advantages and weak points.
Perhaps the oldest mounting for a concave grating is the so-called Rowland circle configuration. Here, the surface of the grating and an input slit for providing incident light to the grating are positioned on the circle defined by the concave surface of the so-called Rowland grating. With this type of grating, a spectrum is formed on a focal surface which also lies on this circle, known as the Rowland circle. In this manner, a detector or array may be placed at the focal surface to detect, for example, a number of discrete wavelengths. Alternatively, if desired, the configuration may be modified to act as a monochromator, by putting a second output slit at that point on the focal surface on the Rowland circle where light of a desired wavelength is focused.
Another popular mount is the so-called Seya-Namioka configuration where an inlet slit provides light to a focusing aberration corrected grating, with a desired wavelength being detected at an output point typically comprising an outlet slit and photodetector. Wavelength selection is achieved by rotation of the grating about its axis.
Still yet another configuration for using a concave grating is the Wadsworth mounting, in which the grating is illuminated by collimated light. In the Wadsworth mounting, light from an inlet slit is collimated by a large concave mirror and caused to fail upon the grating. The grating then creates a spectrum which is positioned on a focal surface at a distance of approximately half the radius of curvature of the grating.
As can be seen from the above, various spectrographic systems typically operate with an inlet slit and various types of output configurations such as slits, array detectors, or the like. Naturally, merely analyzing light, divorced from a particular physical system, is of limited interest. Rather, the applications of spectrographic analysis are of primary importance. Such applications include passing light through a sample of material and noting the emission spectra in the form of Raman, fluorescence or similar effects. In addition, a sample may be excited by energy other than light, such as electrical energy.
In any case, present industrial practice generally involves the excitation of a sample and the focusing of the emitted light onto the inlet slit of a monochromator or spectrometer. The sample can be a blood sample, a tissue sample, an oil sample or any of these materials in a desired emulsion or solution, or other sample prepared in accordance with techniques known in the art.
In a typical Raman application, light from a solid state laser diode having a bundle diameter on the order of one or two millimeters is used. This "pencil" of light may be used directly or expanded using appropriate optics to a wider collimated pencil of light. This pencil of light is caused to fall upon a sample causing emission of scattered light which is collimated by a concave lens into a relatively wide bundle, typically having a dimension on the order of ten millimeters in diameter. This bundle is sometimes caused to pass through a holographic notch filter where the wavelength of the excitation light is removed. After this the light is focused by a convex lens onto the inlet slit of a monochromator which takes any one of the numerous popular monochromator configurations.
The use of the above filter for filtering out the excitation wavelength, while a common practice today, introduces numerous distortions into the system because of the need for double optics for collimating the scattered light and refocusing it after collimation. However, such collimation is required on account of the fact that the holographic notch filter, in order to operate properly, must receive collimated light. More particularly, such holographic notch filters comprise a volume phase hologram inside a thin holographic film. Volume phase holograms operate on an interference principle in which numerous internal planes with known separations therebetween operate to create destructive interference of light at precise wavelengths. For example, the result of such interference may be substantially 100% reflection for a filter designed to operate at 536 nanometers, within a narrow bandwidth of only two nanometers.
However, in accordance with Bragg's law, interference is a function of the distance encountered by light passing through one plane onto another. The encountered distance varies depending upon the angle of incidence to the filter surface. Thus, there will be a wavelength shift proportional to the angle of incidence to the hologram. Thus, if one wishes to remove a particular wavelength, the relationship of angles between the incident light which one wishes to pass and the incident light which one wishes to reject are ideally the same and the distance between the planes in the volume phase hologram which comprises the notch filter is selected for substantially complete reflection. This is particularly important in view of the fact that the amplitude of light at the source may be on the order of a million times the magnitude of the emitted signal. Some idea of the difficulty of the problem involved in this design may be seen when one considers that oftentimes a sample is illuminated with light at, for example, 536 nanometers and one desires to measures light emitted at a wavelength of 540 nanometers.